One thing I have not seen mentioned regarding idle problems is the age and wear on the throttle body throttle shafts and housing. When you have a vehicle with over 200,000 miles worth of wear on the throttle body you can have some large slop between the throttle shaft and the throttle body. Worst one I saw was almost a sixteenth of an inch of play! So, you're getting air sucked in to the intake that doesn't pass through the airflow meter, and the amount of air can vary depending on how the throttle closes. If it gets too worn the throttle plate won't always return to the same spot so sometimes it idles really fast, other times not. Might be a good idea to wiggle the throttle shaft and see what happens if you have idle issues . . . . 
Tom
84 Westy auto "Spiny Norman" (or "Panzerkampingwagen" depending on the mood I'm in)
Oh, and a '98 Mercedes E320, a '86 Alfa Spider and a '87 Maserati 425. Amazingly the Alfa is the most reliable. We won't talk about the Maserati ;~) Road and Track said about the Biturbo (applies to 425) that "Biturbo" in Italian translates as "Expensive Italian Junk."